1. Field of the Invention
The present invention relates to a battery pack to be mounted to an electric vehicle or the like. Specifically, the present invention relates to a battery pack comprising a plurality of rectangular shaped batteries formed into a thin rectangular-parallelepiped shape which are stacked side-by-side in a thickness direction to be integrated into a battery pack.
2. Description of the Related Art
A secondary battery which can be repeatedly charged and discharged is used as a power source of a motor in various types of electric vehicles, for example, a hybrid electric vehicle, or the like. A secondary battery, when used in an electric vehicle, is required to have a large capacity of charge and a high voltage. Therefore, a plurality of thin rectangular-parallelepiped shaped batteries, each comprising a plurality of battery cells, are stacked side-by-side and integrated to form a battery pack. The battery pack is mounted in a vehicle.
FIG. 8 is a perspective view of an example of a battery pack 800 to be mounted in an electric vehicle. The battery pack 800 shown in FIG. 8 comprises a plurality of rectangular shaped batteries 801, each having a thin rectangular-parallelepiped shape. The rectangular shaped batteries 801 are stacked side-by-side in the thickness direction. All the rectangular shaped batteries 801 stacked side-by-side are fixed to a lower case 863 with each being pressurized toward the thickness direction by a pair of pressurizing plates 830 located on both ends of the stacked rectangular shaped batteries 801 in the thickness direction.
Each of the rectangular shaped batteries 801 comprises Nickel-metal hydride secondary batteries having similar structures and a thin rectangular-parallelepiped shape battery case. An inner space of the battery case is divided into, for example, six battery cells located in parallel in the width direction, which is orthogonal to the thickness direction and a height direction, by five walls extending in the thickness direction. Each of the battery cells contains a plurality of positive plates and a plurality of negative plates. In the battery cells, the positive plates and the negative plates are provided with separators interposed therebetween.
All the positive plates in the battery cell are connected to a collector plate. All the negative plates in the battery cell are also connected to a collector plate. In the battery cell, the collector plates oppose each other with all the positive plates and the negative plates interposed therebetween. The collector plates extend along side surfaces of the battery cases. The collector plate of the positive pole located in one battery cell is connected to the collector plate of the negative pole located in the adjacent battery cell. In the battery cells located on both ends of the battery case in the width directions, the positive collector plates and the negative collector plates extending along the side surfaces of the battery case are respectively connected to positive and negative posts 818 which are respectively provided on the side surfaces of the battery case with washers 809.
The rectangular shaped batteries 801 are positioned so that the positive posts 818 of rectangular shaped batteries 801 and the negative posts 818 of the adjacent rectangular shaped batteries 801 are aligned alternatively in the thickness direction. Furthermore, all rectangular shaped batteries 801 are connected in series so that the positive post 818 and the negative post 818 adjacent each other are electrically and mechanically connected to each other by a bus bar 861. The bus bars 861 are fixed by nuts 862 which are screwed to the positive and negative posts 818.
The pair of pressurizing plates 830 for pressurizing all the stacked rectangular shaped batteries 801 are connected to each other by an upper pair of tie rods 850 and a lower pair of tie rods 850 with all the rectangular shaped batteries 801 being pressurized. Each end of the tie rods 850 is fixed to the respective pressurizing plates 830.
The lower case 863 has a bottom surface 863a which supports the rectangular shaped batteries 801 and the pair of the pressurizing plates 830, and a side surface 863b extending upwards from one end of the bottom surface 863a. Each of the rectangular shaped battery 801 is respectively fixed to the bottom surface 863a of the lower case 863 by a pair of bolts 864.
In a battery case 800 having the above-described structure, upper portions of the pair of the pressurizing plates 830 are connected to each other by a pair of the tie rods 850. Similarly, lower portions of the pair of the pressurizing plates 830 are connected to each other by a pair of the tie rods 850. By adjusting the tie rods 850 so that the pressurizing plates 830 locate close to each other, all the stacked rectangular shaped batteries 801 are maintained under pressure. The rectangular shaped batteries 801 in such a state are fixed to the bottom surface 863a of the lower case 863 by the pair of the bolts 864 and form the battery pack 800. The battery pack 800 formed as described above is mounted to be used in an electric vehicle, or the like.
While the rectangular shaped batteries 801 of the battery pack 800 is used, the temperature inside the rectangular shaped batteries 801 rises and hydrogen gas is generated due to an electrochemical reaction which occurs in the battery cells. Thus, the battery cases, which are made of a synthetic resin, expand. Creep deformation of the battery cases of the rectangular shaped batteries 801 occurs when they are maintained pressurized for a long time under a high temperature by the pair of the pressurizing plates 830 connected to each other by the upper pair and the lower pair of tie rods 850. After use of the rectangular shaped batteries 801, the battery cases of the rectangular shaped batteries 801 which suffer the creep deformation are left in a low temperature atmosphere. Thus, the rectangular shaped batteries 801 which suffer the creep deformation shrink and a gap may be generated between the rectangular shaped batteries 801 adjacent each other.
Since the above-described battery cases of the rectangular shaped batteries 801 are fixed to the bottom surface 863a of the lower case 863 by the pair of the bolts 864, and the posts 818 of the rectangular shaped batteries 801 adjacent each other are connected by the bus bar 861, there is no possibility that all of the rectangular shaped batteries 801 are disconnected from each other. However, such a structure in which all the battery cases of the rectangular shaped batteries 801 are fixed to the bottom surface 863a of the lower case 863 by the pair of the bolts 864 requires screwing a number of threaded bolts 864. Thus, there is a problem that the bolting cannot be readily performed.
If a gap is generated between the rectangular shaped batteries 801 adjacent each other, loads imposed by the tie rods 850 are not applied to the battery cases of the rectangular shaped batteries 801 any more. As a result, expansion of the battery cases of the rectangular shaped batteries 801 cannot be prevented thereafter.